1. Field of the Invention
This invention most generally relates to pad transfer printing pads. Even more particularly to pad transfer printing pads used on contact lenses. Even more specially to pad transfer printing pads used with a special ink or media to render the iris part of contact lens opaque.
2. Description of the Prior Art
Pad transfer printing has been in use for over 100 years. However, new inks and print pads must be developed as new articles of manufacture present new substrates and non-flat surfaces to be imprinted. Printing is no longer done on just paper or just flat surfaces or even hard surfaces. Such is the case with contact lenses. The lenses are delicate and require a printing pad which is soft so as not to damage the lens during the printing cycle. The print pad must also be able to resist any chemical action of the ink, be adequately resilient to "remember" its shape when it picks up more ink for the next impression, and be of such durability to withstand hundreds of print cycles in order to be economically feasible.
The prior art was not that instructional. U.S. Pat. No. 5,142,722 by Kolb, Transfer Printing of Furniture End Pieces, teaches heat transfer of sublimation decals at high pressure and high temperature (in the range of 400.degree. F.). This method is not suitable for contact lens printing. U.S. Pat. No. 5,193,457 by Hahn et al., Process for Printing on a Glass Sheet with a Decorative Frame, teaches a process of silk-screening onto glass. U.S. Pat. No. 5,270,507 by Nakamura et al., Push Button Switch and Method for Manufacturing Same, teaches heat transfer printing to rubber materials at temperatures in the range of 150.degree. C. to 230.degree. C. U.S. Pat. No. 5,280,006 by Beck et al., Thermal Transfer Printing Receiver, relates to printing by transferring frown a dyesheet to a receiver in response to a thermal stimuli. As can be readily seen, the prior art has no relevance with the current invention.
Initially, contact lenses were tinted when the wearer wanted to enhance the iris color of his or her eyes. However, until very recently, printing on the contact lens to such an extent as to change the wearer's iris color entirely was not possible. The ink, or media, had to be opaque and be placed on the side of the contact lens that comes into contact with the eyelid. The industry was slow to find such a media with both characteristics of opacity and safety.
An opaque ink was specifically created for contact lenses and covered in U.S. Pat No. 4,668,240 by Loshaek, Pigment Colored Contact Lenses and Method for Making Same. The ink can tint a lens or make it opaque depending on the dye used. When the ink is dry on the contact lens, it is inert enough to have been approved by the FDA to be worn next to the eyelid. However, when wet, it caused the print pads' printing quality in use at the time to disintegrate much faster than normal because of swelling.
The print pads used previously for pad transfer printing could not be used with the new ink because it destroyed the pad transfer printing pads too quickly to be economically feasible. New print pads were developed, but these improvements still had problems. In the automated system, the ink stays on the print pad long enough to start a chemical reaction with the pad. The print pads that are currently in use do not last more than three to four hours of continuous printing. This translates to approximately 1,200 impressions before the print pads are no longer usable and must be replaced. The time used to stop the printing machine and replace the pad transfer printing pads is lost time and must be charged to the overall cost of manufacturing printed contact lenses. There is also a problem over time with the print pad swelling as a reaction to the ink. When the print pad swells, the printing precision is compromised.
Another problem with the print pads previously used was the poor adhesion. The old print pads would transfer the ink to the lens, but the ink would not always adhere permanently to the lens.
It would be desirable and advantageous to create a pad transfer printing pad soft enough to be used for contact lenses that would accept the ink from a source and release the ink freely to the lens (more affinity than the ink source but less affinity than the lenses), that would last for more than three hours on an assembly line (or more than 1,200 transfers), and still retain all the positive aspects of the prior art. The savings of such a pad transfer printing pad would be twofold: an initial savings for a reduced number of print pads to make the same number of contact lens impressions, and secondly, the times saved on a work shift that previously was used to change the print pads at least twice during every shift (some shifts now require three changes).
It would be desirable if the improved print pad provided good adhesion of the ink to the contact lens.
It would also be desirable and advantageous if the greatly improved print pads cost no more than, or even less than, the current state of the art pad transfer printing pads. This additional cost cutting measure would result in a threefold savings over the prior art.
It would also be desirable if the print pads had an improved resistance to swelling caused by the ink. This attribute would enhance the print quality.